10:20   Control of laminar-turbulent transition I Wall-cooling for transition delay in crossflow-dominated boundary layers Marina Barahona, Alberto Felipe Rius-Vidales, Marios Kotsonis Abstract: This work will present results where the application of TSP is used to investigate thermal flow control. The focus will be on studying the impact of wall cooling in laminar-to-turbulent transition driven by stationary crossflow instabilities (S-CFI). For that purpose, a detailed parametric study is conducted, varying the cooling temperature ratio and DRE forcing parameters (amplitude and wavelength), with TSP measurements used to obtain accurate transition front location. Complementary HWA measurements, employing a V-wire probe configuration, will also be presented. The combined use of two-component HWA and TSP provides a comprehensive experimental dataset, enabling precise identification of the transition front and detailed analysis of both steady and unsteady perturbations in the streamwise (u) and spanwise (w) velocity components. The Effect of a Surface Hump on Swept Wing Transition Under Varying Flow Conditions Marco Radaelli, Alberto Rius Vidales, Marios Kotsonis Abstract: Laminar-turbulent transition of the flow over an aircraft remains a major contributor to the aerodynamic drag produced by an aircraft. For swept wing, commonly used in commercial flight, this transition process is driven by stationary crossflow instabilities (CFI). In recent years, several numerical and experimental investigations have attempted to understand and to control this phenomenon, such that transition is delayed. Consequently, this would result in less produced aerodynamic drag. The Flow Control research group at TU Delft has investigated the possibility of using a step as a passive laminar flow control (PLFC) device, after unexpected experimental observations by Rius-Vidales and Kotsonis (2021), showing transition delay over such a geometry. Numerical work by Casacuberta et al. (2022) supports these observations by showing that a step is not universally destabilizing CFI, after which Casacuberta et al. (2024) propose mechanisms explaining these observations. The concurrent development of a numerical Harmonic Navier-Stokes (HNS) solver by Westerbeek et al. (2024) provides a robust way to explore the influence of surface modifications on crossflow, at a fraction of the cost of DNS. The wish to extend the applicability of stabilising geometrical features has lead to the proposition of using a smooth surface hump. Rius-Vidales et al. (2025) show experimentally that smooth surface features, such as the hump, delay transition, albeit under certain conditions. Once the hump operates outside these conditions, it suddenly triggers transition at its location. These observations, showing great sensitivity of the hump to the flow conditions, currently lack a clear understanding of the mechanisms at play which cause either transition advancement or delay, as well as a well-documented envelope of the operational conditions of the hump. The objective of this study is to explore and map the latter, by means of an experimental parametric study. This study aims to be a foundation for further research on the investigation of the mechanisms that manifest across the whole operational space of the hump. Transition Path Alteration under Plasma Actuation over a Swept Wing Takashi Matsuno, Rikiya Hata, Ikuya Yoshimi, Taichi Kawai Abstract: This study investigates the effect of plasma actuator-induced crossflow suppression on transition mechanisms over a swept wing. Wind tunnel experiments reveal that plasma actuation delays transition by suppressing high-frequency Type I secondary instability. However, transition still occurs via low-frequency Type III modes. The findings highlight a shift in dominant instability pathways and demonstrate that complete crossflow cancellation is necessary for enhanced control effectiveness.

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